Animals on Drugs

Antibiotic-Pumped Livestock May Be Breeding Superbugs
Lisa Blackshear

Slowly, but surely, bacteria are winning. Though they replicate as often as dozens of times a day, bacteria face only about 150 types of antibiotics approved for human use in the past 60 years. Misuse of antibiotics in treating human infections has clearly contributed to the rise in drug-resistant bacteria of all types, but now the use of antibiotics in agriculture has also been recognized as a likely contributor.

Of the more than 50 million pounds of antimicrobial drugs produced in the United States to kill bacteria, fungi and viruses, about half are used in agriculture. In addition to the drugs’ obvious role in treating sick animals and plants, low doses are applied to protect crops, and given to healthy animals, including farm-raised fish, to make them grow faster with less feed.

“There’s enough evidence that their use increases [antibiotic] resistance of bacteria in animals,” warns Dr. Tamar Barlam, director of the Antibiotic-Resistance Project of the Center for Science in the Public Interest. “That bacteria can then infect people, and jump to other organisms that are in humans.” The Center joins a coalition of other concerned groups calling on the U.S. Food and Drug Administration (FDA) to immediately ban the agricultural use of such important human antibiotics as penicillin, tetracycline and erythromycin, and to refuse new approvals of human antibiotics for growth promotion or disease prevention in animals.

Currently, pharmaceutical companies are not even required to disclose data on antibiotic use in agriculture. “There’s no good information out there to see how many [applications] of antibiotics are used in animals,” said Steven Fondriest, a Union of Concerned Scientists spokesperson. “The FDA can rescind [approval of] drugs if it feels there is a threat to human health,” said Barlam. “But the data they need to prove it is hidden.”

Darwin’s Finest—and Fastest

Bacteria make good models of natural selection. Since they replicate quickly, any small change in their make-up that allows them to survive better will soon spread through generations. But this bodes badly for infected people—particularly if the newly acquired bacterial trait is an immunity to antibiotic drugs. Without competition from their more susceptible relatives, the numbers of resistant bacteria grow, increasing the potential for human infections that do not respond to conventional treatment.

In 1997 that scenario became reality for nine family members of a Vermont dairy farm, the first in the U.S. to be infected with a particular antibiotic-resistant strain of Salmonella. The bacterial infection almost killed the farmer and caused the death of 13 of her cows before it was identified as a strain resistant to five types of antibiotics. Researchers concluded the family became infected by contact with sick cows and by drinking unpasteurized milk.

Veterinarians began using antibiotics to treat animals soon after penicillin and other antibiotic drugs were discovered in the 1930s. When researchers in the 1950s discovered that feeding food animals low levels of the drugs increased their growth and weight gain, the practice became widespread. As much as 90 percent of the antibiotics applied to animals in the U.S. are now used to promote growth and to prevent disease, rather than to treat a specific ailment.

Some researchers theorize that low doses of antibiotics may also induce weight gain by killing naturally occurring bacteria in the animal’s gut, allowing more nutrients to be passed along. The practice has been particularly useful for “factory farms,” which seek to maximize profits by keeping more animals in less space, increasing their exposure to bacteria.

Still Multiplying

Studies have shown the widespread use of antibiotics in livestock coincided with a rise in antibiotic-resistant strains of bacteria, including Salmonella, Escherichia coli (better known as E.coli) and Campylobacter. Antibiotic-resistant bacteria can also pass the trait of genetic resistance to other types of bacteria nearby.

In 1970, a FDA task force confirmed there was reason to be concerned. It concluded that “subtherapeutic” (low, constant) use of antimicrobials “favored the selection and development of resistant bacteria,” and that use of antimicrobial drugs had caused multi-drug resistant bacteria in animals. It also found that treated animals may serve as “a reservoir of antibiotic-resistant pathogens.”

The agency proposed to withdraw approval for long-term animal use of two important human antibiotics, penicillin and tetracycline, in 1977. But after agricultural industry groups criticized the proposal, the FDA allowed the use to continue. It then approved fluoroquinolones for use in poultry, by prescription and for therapeutic uses only, in 1995. Fluoroquinolones are used to treat a wide range of human infections, particularly foodborne infections resistant to other antibiotics.

As some feared, researchers recently reported seeing an increase in fluoroquinolone-resistant Campylobacter species in U.S poultry. Campylobacter is the most common diarrheal-causing bacteria in the U.S., usually acquired from contact with raw poultry or by eating the undercooked meat. Several European countries that permit fluoroquinolone use have reported subsequent increases of resistant bacteria species as well.

In response, the FDA has proposed a human risk assessment and management framework for future evaluation of antibiotic use in animals, but has not withdrawn approval for any long-term, low-level drugs currently in use. “The FDA is in a difficult position,” says Barlam. “It can’t subpoena information and it’s under a lot of pressure from congressmen in agriculture states to not act in a definitive way.”

The European Union, on the other hand, voted last year against administering four antibiotics similar to human drugs in animal feed. The action was the result of strong lobbying by Sweden, which had eliminated subtherapeutic use of antibiotics in food animals in 1986, and was supported by the World Health Organization (WHO).

The WHO issued new recommendations this past June for the “containment of antimicrobial resistance,” which include: rapid phase-out of human antimicrobials used for animal growth promotion, national monitoring systems and prescription requirements for all antimicrobials used to treat disease in food animals.

Common Kitchen Sense

Most recently, concern about antibiotic resistance has been raised around the hotly debated issue of genetically engineered (GE) food. Foreign genes are frequently spliced together using antibiotic resistance marker genes, which some fear may recombine with bacteria in the environment or in people or animals that eat such food. “The increase in antibiotic resistance is distressing to say the least,” agrees Abigail Saylors, a professor in the Department of Microbiology at the University of Illinois-Urbana-Champaign. But “if someone wants to know what they can do to protect themselves from antibiotic resistance,” she adds, “the best thing to tell them is not ?don’t eat GE crops,’ it would be ?cook the chicken


Another option, of course, is to buy an organic chicken, or other organic meat or dairy products that do not allow the use of antibiotics. While these animal products may still have bacteria levels comparable to those raised with antibiotics, and likewise require careful handling, organic livestock may be less likely to carry drug-resistant strains or have antibiotic residues. Granted a “Certified Organic” label by the U.S. Department of Agriculture just last year, however, organic livestock still only accounts for less than one percent of the U.S. total.

New antibiotic products, including hand soaps, plastics, toothpaste and cutting boards, use an antimicrobial agent call triclosan. Unlike bleach and soap that destroy and dislodge germs, triclosan works by interfering with a specific bacterial enzyme, which not all bacteria have. Researchers now say triclosan could encourage the proliferation of drug-resistant bacteria, and the American Medical Association has urged government agencies to increase regulations on antibiotic products. Some exposure to germs helps build a healthy immune system, physicians say, but the best germ-fighting measure is still to do as your mother told you, and wash your hands.